Not an answer, but see Baba Batra 9B: "Jeremiah said to the Holy One, blessed be He: Sovereign of the Universe, even at the time when they conquer their evil inclination and seek to do charity before Thee, cause them to stumble through men who are not fitting recipients, so that they should receive no reward for assisting them." -- halakhah.com/bababathra/bababathra_9.html
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MenachemSep 22 '11 at 6:27

3 Answers
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R' Yehuda Tzvi of Rozla was once visited by a Chassid who gave him a kvittel (prayer request) with the accompanying traditional pidyon (money given to a Tzaddik).

The Rebbe asked him (rhetorically) how he, the Rebbe, was allowed to receive a pidyon; after all, the money is only given on assumption that he is a Tzaddik. As he (the Rebbe) knows his own worth, how could he take money given to him on false assumptions?

The Rebbe answered that there is a Gemarah which says that Yirmiyahu cursed the people of Anasos that whenever they wanted to give charity, they should give it to undeserving people.

One could ask, why did Yirmiyahu curse the Jews? It would be hard to believe that he hate those Jews so much.

Rather, the world operates on the principle of Isarusa Dile'ela (an awakening from above) is dependent on an Isarusa Diletata (an awakening from below). Now the situation of the Jews of Anasos was hopeless. They didn't deserve to be saved. The only hope for them was supernal kindness not dependent on the merits of the recipients. Therefore, the only way to get such supernal kindness is through giving to people not deserving of kindness.

Now the Rebbe answered his question. Through the Chassid's giving charity to an undeserving person, he (the Chossid) will be saved even though he is undeserving.

The Ben Ish Chai in Shu"t Rav Poalim (Sec. 4. Orach Chaim - Siman 2) says in the context of where a person unknowingly wore invalid teffilin for many years. The Ben Ish Chai writes that the person still gets the merit for the mitzvah. He quotes the Chid"a (Sefer Devash Ma'areches 4 - Sec. 4) who echoes this point.
(end of reference)

(Perhaps the Gemara in Kiddushin (40a) is a proof as well, which states:
אמר רב אסי אפילו חשב אדם לעשות מצוה ונאנס ולא עשאה, מעלה עליו הכתוב כאילו עשהו
. If one desired to fulfill a mitzvah but was unable to due to extenuating circumstance, the Torah considers as if he fulfilled it.
So too, as when you gave the money you desired to give tzedaka, the fact that it was fraud is an Ones and you had no idea it was a fraud. Therefore, you still get the merit of giving charity.)

That is in regards to merit. However, in terms of fulfilling your obligation, it would seem to be that you have not fulfilled the mitzvah of Ma'aser Kesafim, for this is not a case of Ones, physically barred from performing the mitzvah, rather this would be considered a case of shogeg - an unwitting mistake, as you should have looked into the charity beforehand. (As per this source quoted by Shalom here:)

Furthermore, from a Nedarim - halachic oath standpoint, you would be obligated to give again, for if you had in mind to pledge to give money to a specific yeshiva or organization. Because of there was fraud, the money never made it to that organization. Thus, in this case you would need to give the money again in order to fulfill the oath that you took.